test(pkg): Extend the test to use recursive submodules

[Leonidas-from-XIV]

An end-to-end test to make sure nested submodules work as well. In this case it is mock-repo which references a submodule packages which contains a direct package foo and another submodule bar which contains the bar package. The solver tries to find a solution for bar and succeeds.

Fixes #9377

[rgrinberg]

Could you move this to a separate test case? I know it's a little more work, but it's getting hard to read everything inside opam-repository-download.t

[Leonidas-from-XIV]

Yes, that's a good point, I moved the whole submodule test to its own test.